1. Field of the Invention
The field of the invention is that of filtering in electromagnetic waveguides. More specifically, the invention relates to a device for the filtering of waves propagating in rotational symmetrical waveguides, such as the circular waveguides or coaxial waveguides used in TE.sub.11 mode.
The invention can be applied in particular to dual band filters. A major use of this invention is indeed the making of dual band and bi-polarization duplexers, notably when the ports of the duplexers are in the same rectangular guide standard. This is, for example, the case of the 10.95-12.5 GHz and 14-14.5 GHz bands in the WR 75 waveguide. As a general rule, the horizontal and vertical polarizations of these duplexers are not identical in the two frequency bands considered.
2. Description of the Prior Art
FIG. 1 shows a schematic view of a known type of duplexer such as this. The high (for example 14-14.5 GHz) band port 11 and the low (for example 10.95-14.5 GHz) band port 12 are constituted by rectangular waveguides. The output 13 towards the radiating element is constituted by a circular waveguide.
For the low band, the excitation is achieved by a coupling, by means of a slot in the duplexer 14, between the rectangular guide and the circular guide. If the wave is to be propagated towards the radiating element and not towards the high band port 11 constituted by a rectangular guide, it is necessary to place a rectangular-to-circular transition 15 and a polarization filter 16 between the high band 11 port and the duplexer 14.
There already exists several known types of polarization, such as those shown in FIGS. 2 and 3. FIG. 2 shows a filter with metal plate 21, and FIG. 3 shows a filter with metal wires 31, 32, 33. These metal elements 21 or 31, 32, 33, selectively placed in the waveguide 22, enable the elimination of a polarization, hence the elimination of a given frequency band.
An error of one degree in the positioning of this plate 21 or of these wires 31, 32, 33 causes a maximum decoupling (transmission of the electromagnetic wave from the port 11 to the port 12) of 35 dB, which is generally insufficient. In mechanical terms, therefore, the making of these devices calls for high precision in the placing and fixing of the plate 21 or of the wires 31, 32, 33 to the interior of the circular guide 22. Furthermore, the manufacture of such filters calls for several successive and delicate steps.